FSN.getAdditionalBlock() consists of two parts I and II. Each part calls analyzeFileState(), which among other things check replication of the penultimate block via checkFileProgress(). See details in
Checking file progress in Part II is not necessary, because Part I already assured the penultimate block is complete. It cannot change to incomplete, unless the file is truncated, which is not allowed for files under construction.
HDFS-9690 ClientProtocol.addBlock is not idempotent after HDFS-8071